In general, a semiconductor chip and an external circuit are mainly electrically connected with each other by wire bonding. However, in the case where the semiconductor chip is used for operating at a high frequency, resistance and inductance components due to a wire operate as undesired elements in a circuit operation. When several ground pads exist in the circuit in the semiconductor chip and are grounded through wire bonding, oscillation due to a minute difference between ground voltages in the ground pads occurs in some cases. Since a ground pad for wire bonding is additionally disposed at the edge of the semiconductor chip in order to minimize a length of the wire, an area of the chip increases and the chip cost rises.
Meanwhile, in the case of the semiconductor chip such as the power amplifier or the power semiconductor, the semiconductor chip is required not to operate at a high temperature by rapidly discharging heat generated from the semiconductor device outside the semiconductor chip. The heat generated from the semiconductor device is generally discharged through the semiconductor substrate, but in the case of a semiconductor substrate having high thermal resistance, the heat is not sufficiently discharged to increase a temperature of the semiconductor chip, and as a result, the operation thereof is unstable. In order to solve the problem, a metal having a better heat transfer characteristic than the semiconductor substrate is thickly formed at a part of the front side of the semiconductor device to act as a thermal shunt in some cases, but a heat discharging path needs to be made at the front side of the semiconductor chip so as to discharge heat accumulated in the thermal shunt at the front side outside.
To this end, in order to electrically and thermally connect the semiconductor chip and the outside, the best method is to connect a circuit formed at the front side of the semiconductor chip to the rear side through the via hole with a metal. In particular, in the case of a power amplifier operating at a high frequency, it is most preferable to form a ground through the rear side so as to have the shortest path in order to stabilize an electric characteristic. It is most efficient when heat generated in the semiconductor chip is discharged by a module packaging the semiconductor chip through the via hole of the rear side of the semiconductor substrate.
In the case of a semiconductor device and an integrated circuit manufactured on a semiconductor substrate (hereinafter, referred to as a “compound semiconductor substrate”) made of a compound, the via hole needs to be formed at the compound semiconductor substrate. The compound semiconductor substrate includes GaAs, InP and SiC substrates and the like, and a deep etching technique for forming the via hole is not completed as compared with a general Si substrate. In particular, in the case of the SiC substrate, the maximum etching speed is 2 μm per minute in a current technique. Accordingly, if the thickness of the semiconductor substrate may be formed thin, it is possible to largely overcome difficulty in a process for etching the via hole. As the thickness of the semiconductor substrate becomes thinner, heat transfer resistance through the semiconductor substrate becomes smaller, and thus the heat discharge is easy, to thereby largely contribute to stability of the semiconductor chip.
As described above, according to the related art, the via hole is formed at the rear side by etching the semiconductor substrate having a thickness of about 100 μm, but in a compound semiconductor substrate made of a specific material, etching itself is difficult, or although the etching is possible, etching over a long period of time causes the thermal damage to the semiconductor chip or breakdown in equipment.